I had the same problem and it was getting worse. I took out the EGR valve turned it updide down and filled it up with WD-40. I let this sit for a couple of hours. I also spayed the ports (2) on the engine with WD-40. I used quite a bit. I put EGR back and started down the street. It ran very rough at first and it blew out alot of blue smoke. I ran it hard and it cleaned out the EGR ports. This totally fixed the bucking problem. I am running smooth at all speeds!